This invention relates to circuit fabrication, and in particular to a method of providing chemical and mechanical protection of circuit components.
Micro-electronic circuits, such as thin film and hybrid integrated circuits, have found increasing use in sophisticated systems where it is necessary to assemble the circuits on printed wiring boards by various soldering techniques. The need for precision and dependability of such circuitry has necessitated an effort to provide the greatest possible protection of circuit components from contaminants in the atmosphere as well as from solvents used in cleaning the printed wiring boards after assembly.
It has been found that room temperature vulcanizing (RTV) silicone rubber encapsulants provide good protection against moisture, particulate matter and in some cases light. However, such encapsulants provide little resistance to solvents and other chemicl solutions typically used to deflux the printed wiring boards after the soldering operations. Thus, if the circuit is exposed to such liquid solvents and chemicals for more than one minute, it was found that the encapsulant could be penetrated causing damage to silicon integrated circuit chips and crossovers. In addition, the encapsulant does not provide sufficient mechanical and handling protection for delicate circuit components. In most cases, either a plastic housing or a metal cover is therefore desirable for mechanical protection. Although the housings and covers achieve this function, they provide no protection against the solvents and chemicals which penetrate the encapsulant.
Many prior art schemes exist for providing a second layer over the encapsulating layer to enhance the protection of circuit components. (See, for example, U.S. Pat. No. 3,441,813 issued to Takatsuka, and U.S. Pat. No. 3,566,208 issued to Wang.) It has been found, however, that when conventional teaching was followed, the second layer would not adhere well to the encapsulant. Thus, the cover coat would easily separate from the encapsulant when a slight lifting force was applied or when the circuit was subjected to thermal cycling.
It is therefore an object of the invention to provide enhanced chemical and mechanical protection for circuit components.
It is a further object of the invention to provide such protection with a dual layer of encapsulant and cover coating which adhere strongly to one another.